On the multiplexer, you have several signals: 4 input bits, 2 select bits and an output. For these sorts of problems, you have to hard code the input bits and select bits of the multiplexer as either the inputs (W, X, Y, Z) or 0 or 1. I did out the truth table for this function and chose W and X to be my select bits with W as the most significant bit. Comparing with the truth table, I found the input bits, from MSB to LSB, to be 1, Z, 0, Y.